UTRAN (Universal Terrestrial Radio Access Network) is a term identifying the radio access network of a WCDMA based UMTS (Universal Mobile Telecommunications System), wherein the UTRAN consists of Radio Network Controllers (RNCs) 13b and NodeBs 12b i.e. radio base stations as illustrated in FIG. 2. The NodeBs communicate wirelessly with mobile user equipments (UEs) 14b and the RNCs 13b control the NodeBs 12b. The RNCs 13b are further connected to the Core Network (CN) 10b. Evolved UTRAN (E-UTRAN) is an evolution of the UTRAN towards a high-data rate, low-latency and packet-optimised radio access network. Further as illustrated in FIG. 1, the E-UTRAN consists of radio base stations (eNBs) 12a, and the eNBs are interconnected and further connected to the Evolved Packet Core network (EPC) 10a. E-UTRAN is also being referred to as Long Term Evolution (LTE) and is standardized within the 3rd Generation Partnership Project (3GPP). FIG. 1 also shows UEs 14a in communication with the eNBs 12a. 
Existing WCDMA (Wideband Code Division Multiple Access) based UTRAN utilize single carrier frequency transmission comprising of 5 MHz in each direction i.e. downlink and uplink. Similarly the E-UTRAN also uses single carrier frequency transmission with a bandwidth which may be between 1.4 MHz to 20 MHz. The evolution of WCDMA towards multicarrier WCDMA and of E-UTRAN to LTE advanced comprising multicarrier transmission is underway. Hence this ongoing evolution is the possibility of transmitting data over more than one carrier at a time. One of the fundamental objectives of this evolution is to enhance data rate per user while still maintaining robust mobility performance. Therefore the introduction of additional carriers also requires certain level of modification in the mobility principles and radio resource management algorithms controlling the mobility.
In WCDMA single carrier system the following three downlink neighbour cell measurements are specified primarily for mobility purpose; CPICH RSCP (Common Pilot Channel Received Signal Code Power), CPICH Ec/No; CPICH Ec/No=CPICH RSCP/carrier RSSI (Received signal strength indicator) and UTRA Carrier RSSI.
The RSCP is measured by the UE on cell level basis on the common pilot channel (CPICH) and the UTRA carrier RSSI is measured over the entire carrier. The UTRA carrier RSSI is the total received power and noise from all cells (including serving cells) on the same carrier. The above mentioned CPICH measurements are the main quantities used for the mobility decisions.
In E-UTRAN the following three downlink neighbour cell measurements are specified also primarily for mobility purpose; Reference symbol received power (RSRP), Reference symbol received quality (RSRQ): RSRQ=RSRP/carrier RSSI and E-UTRA Carrier RSSI.
The RSRP or RSRP part in RSRQ in E-UTRAN is solely measured by the UE on cell level basis on reference symbols. As in case of WCDMA, the E-UTRA carrier RSSI is measured over the entire carrier. It is also the total received power and noise from all cells (including serving cells) on the same carrier. The two RS based measurements are indeed also the main quantities, which are likely to be used for the mobility decisions.
In cdma2000 1xRTT system and in cdma2000 HRPD system the pilot strength is specified for quality measurement for mobility. In Wimax IEEE 802.16 systems, WiMAX Preamble CINR and WiMAX RSSI are used for mobility measurements.
WiMax Preamble CINR is the Carrier to Interference and Noise ratio of the WiMAX DL preamble measured by the UE for a particular BS. This measurement quantity provides information on the actual operating condition of the receiver, including interference and noise levels, and signal strength. It therefore depicts the cell quality and is analogous to RSRQ and CPICH Ec Carrier to Interference and Noise ratio/No in E-UTRAN and WCDMA respectively.
Wimax RSSI is the Received Signal Strength measured by the UE from the DL preamble for a particular base station. It corresponds to signal strength measurements (i.e. RSCP or RSRP) in WCDMA or E-UTRAN.
The neighbour cell measurements are typically averaged over long time period in the order of 200 ms or even longer to filter out the effect of fast fading.
There is also a requirement on the UE to measure and report the neighbour cell measurements (e.g. RSRP and RSRQ in E-UTRAN) from a certain minimum number of cells. In both WCDMA and E-UTRAN this number is 8 cells comprising of one serving and seven neighbour cells on the serving carrier frequency or commonly termed as intra-frequency.
CPICH Ec/No and RSRQ are the so-called neighbour cell quality measurements used in WCDMA and E-UTRAN respectively. The explanation is also valid for quality measurements in other systems e.g. WiMax Preamble CINR in Wimax system.
The goal of the neighbour cell quality measurement is to estimate and predict the long term downlink quality that may be experienced by the UE in a particular cell. It should indeed indicate the signal quality or throughput that the UE will achieve in a cell. This prediction enables the UE and the network to choose the most appropriate cell when performing cell reselection and handovers respectively. In E-UTRAN any set of resource blocks (i.e. part of the cell bandwidth) may be assigned to the UE for transmission.
In multicarrier system like in multicarrier WCDMA the overall average and long term quality prediction of all or a sub-set of carriers is important to be known prior to handover or cell reselection.
In both WCDMA (HSDPA) and E-UTRAN, the UE reports a channel quality indicator (CQI), which is expected to depict the instantaneous channel quality. Furthermore CQI is only reported from the serving cell. Therefore the purpose of CQI is to track fast fading and is mainly used for scheduling and link adaptation at the base station. In current HSDPA system only one CQI is reported at a time since there is a single carrier in the downlink and uplink. In E-UTRAN system, the UE may be configured to report the CQI over a part of bandwidth to be able to do frequency domain scheduling.
As stated above, mobility decisions (i.e. cell reselection and handovers related) require long term averaged downlink quality. For the purpose of mobility, the network usually uses neighbour cell quality measurement quantity (i.e. CPICH Ec/No in WCDMA or RSRQ in E-UTRAN), However the use of CQI for mobility decisions is not precluded. For instance the network may filter the CQI to alleviate the effect of fading and use it for mobility aspects e.g. starting or stopping of measurement gaps for inter-frequency measurements.
There are basically two kinds of mobility scenario; Idle mode mobility with cell reselection and connected mode mobility with handover.
The cell reselection is mainly a UE autonomous function without any direct intervention of the network until the change of serving cell has already been performed. However, to some extent the UE behaviour in this mobility scenario could still be controlled by some broadcasted system parameters and performance specification. The handover on the other hand, is fully controlled by the network through explicit UE specific commands and by performance specification.
In both idle and connected modes the mobility decisions are mainly based on the same kind of downlink neighbour cell measurements, which were discussed in the previous section.
Both WCDMA and E-UTRAN are frequency reuse-1 systems. This means the geographically closest or physical adjacent neighbour cells operate on the same carrier frequency. An operator may also deploy multiple frequency layers within the same coverage area. Therefore, idle mode and connected mode mobility in both WCDMA and E-UTRAN could be broadly classified into three main categories: Intra-frequency mobility (idle and connected modes), Inter-frequency mobility (idle and connected modes) and Inter-RAT mobility (idle and connected modes).
In intra-frequency mobility, the UE moves between the cells belonging to the same carrier frequency. This is the most important mobility scenario since it involves less cost in terms of delay as neighbour cells are monitored continuously i.e. not in measurement gaps. The gap assisted measurements as done for inter-frequency mobility involve relatively longer delay compared to those for intra-frequency mobility. In addition, an operator would have at least one carrier at its disposal that it would like efficiently utilize.
In inter-frequency mobility the UE moves between cells belonging to different carrier frequencies but of the same access technology.
In inter-RAT mobility, the UE moves between cells that belong to different access technologies such as between WCDMA and GSM or vice versa.
In order to reduce signalling overheads the UE may be configured to report an event when certain conditions are met. Alternatively the UE will have to report the measurements on serving and neighbour cells continuously.
These reported events are used by the network to take mobility related decisions in connected mode. Furthermore the same event may be reported according to signal strength measurement (e.g. RSRP) or signal quality measurement (e.g. RSRQ) or both as configured by the network. In addition an event may be absolute based on single cell or relative based on comparison between 2 cells (generally between serving and neighbour cells). Typically one or more events are configured and associated thresholds or other parameters are signaled to a UE.
In UTRAN systems some example of events are: Reporting event 1A: A Primary CPICH enters the reporting range, Reporting event 1B: A primary CPICH leaves the reporting range, Reporting event 1C: A non-active primary CPICH becomes better than an active primary CPICH, Reporting event 1D: Change of best cell, Reporting event 1E: A Primary CPICH becomes better than an absolute threshold, and Reporting event 1F: A Primary CPICH becomes worse than an absolute threshold.
Similar events are also specified in other systems. For instance in E-UTRAN the mobility related events are specified in 3GPP TS 36.331 “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification”.
In idle mode no events are reported to the network, but the same measurements may be used for cell reselection as stated above. The parameters are signaled by the network on the broadcast channel. The cell reselection algorithm, which are in some sense analogous to events in connected mode, are specified in the standard to ensure well defined UE behaviour (see e.g. 3GPP TS 25.304 “User Equipment (UE) procedures in idle mode and procedures for cell reselection in connected mode” for UTRAN and 3GPP TS 36.304 “Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode” for E-UTRAN).
In multicarrier systems at least more than one carrier is used in the downlink. One of the multi carriers is called anchor carrier and remaining ones are called supplementary carriers. Other terminologies used in literature for anchor and supplementary carriers are primary and secondary carriers respectively. However, the terminology primary carrier and secondary carrier is used in the remaining description.
The primary carrier contains all physical channels including all common control channels. The secondary carriers may or may not contain all physical channels; for instance it may lack some common control channels. The primary carrier in downlink and in uplink (i.e. if there is more than one carrier in uplink) should work according to legacy operation. Note that legacy operation is based on single carrier.
For instance the UE in dual cell HSDPA (DC-HSDPA) operation, which is being currently specified, is able to simultaneously receive HSDPA traffic over two downlink carrier frequencies. They are also transmitted in the same frequency band from a single serving sector. There is one uplink carver for a DC-HSDPA UE and it is not strictly tied to one of the two downlink carriers. In DC-HSDPA UE primary carrier has all the physical channels including DPCH/F-DPCH, E-HICH, E-AGCH, and E-RGCH. During dual carrier operation in CELL_DCH, the UE secondary carrier is the downlink carrier which is not the UE primary carrier.
In legacy system with single carrier in the downlink the quality based mobility decision are obviously based on the quality estimated on one carrier. In multicarrier systems e.g. in DC-HSDPA, the quality on each carrier may be different due to different interference, adjacent channel interference and noise levels experienced by the UE. The difference in interference is for example due to different level of co-channel interference sources. The difference in noise level is considerably influenced by the frequency band e.g. different carriers may belong to different bands, which may have different sensitivity levels. Nevertheless the co-channel interference remains the dominant factor, which would distinguish the downlink neighbour cell quality on different carriers. Therefore to ensure robust quality based mobility performance the downlink quality on all or at least sub-set of carriers in a set of multicarrier should be taken into consideration in multicarrier system.
The other important family of measurement quantities used for mobility is signal strength based e.g. RSRP in E-UTRAN. This category of mobility measurement is independent of any type of co-channel interference. Therefore signal strength based measurement quantity and the corresponding events from anchor carrier are likely to be sufficient.
The simplest solution is that all mobility based decisions in idle and connected mode are solely based on measurements and events from the primary carrier. The drawback is that this solution does not take into consideration the downlink quality of the secondary carriers, which are though used for transmitting data. Thus in practice the UE could enter or be handed over to a set of multicarrier cell, which do not provide sufficient overall quality when transmitting data. There will be overall loss in user performance e.g. lower than the bit rate envisaged in a multicarrier setup. Therefore full potential of multicarrier system may not always be realizable.
Another solution is that mobility decisions take into account measurements and events from primary as well as secondary carriers. For instance either AND or OR operation could be performed on individual events to generate one aggregate event before deciding to perform handover. Though this solution depicts quality on all carriers in a multicarrier system but the obvious flaw is in terms of increase in the number of measurements and events, which are performed by the UE. On the one hand it increases UE complexity and on the other hand it will increase uplink signalling load and will require more complex processing at the network. The complexity and signalling load will explode with the increase in the number of supplementary carriers. The number of carriers in multicarrier systems (HSPA, IMT-advanced etc) is expected to rise sharply to satisfy the increasing demand of high data rate.
Therefore a solution is needed that provide a reasonable tradeoff between various factors: system complexity, UE complexity, signalling overheads, accuracy of quality measurements etc. in a multicarrier system.